Full Idea
To say that 'if' there are two persons and each person has two eyes there 'will be' four eyes is not a statement of fact, but a statement about the system of numbers which is our own creation.
Gist of Idea
That two two-eyed people must have four eyes is a statement about numbers, not a fact
Source
Charles Sanders Peirce (Scientific Attitude and Fallibilism [1899], II)
Book Reference
Peirce,Charles Sanders: 'Philosophical Writings of Peirce', ed/tr. Buchler,Justus [Dover 1940], p.59
A Reaction
One eye for each arm of the people is certainly a fact. Frege uses this equivalence to build numbers. I think Peirce is wrong. If it is not a fact that these people have four eyes, I don't know what 'four' means. It's being two pairs is also a fact.